crablab, you will need an instructor to conduct the variant differences training for an SEP Class VP aeroplane - and it will need to be signed off in your logbook i.aw. FCL.710(c). Also, if the 17000 hr instructor hasn't flown that variant before, he will indeed need to met the requirements of FCL.710.
However, if he has flown other VP SEP Class aeroplanes before, then he would only need 'familiarisation' on the group aeroplane, which does not actually require an instructor.
It sounds as though the group is being eminently sensible. As for signing-off a pilot who has never flown a VP prop aeroplane before without a flight to show pre-flight CSU checks, take-off and climb, setting up the cruise at the POH RPM/MAP etc. etc..... A slide show cannot really be sufficient, but the flight shouldn't need to take very long.